1. Field of the Invention
The present invention relates to an overcurrent limiting semiconductor device for preventing an overcurrent from flowing across the collector/emitter junction of a transistor.
2. Discussion of the Prior Art
The use of a pair of diodes serially connected across the base/emitter conductors of a transistor is one known way to limit emitter current to less than a predetermined value. Such an arrangement is described in Japanese Patent Application (OPI) No. 81313/83, and is depicted in FIG. 2 of the present application. As shown in FIG. 2, a pair of diodes 22, 23 are serially connected across the base/emitter of a transistor 21, to limit the emitter current of the transistor 21. When current I.sub.R flowing through a resistor 24 having a resistance value R increases to cause the sum of the voltage (R.I.sub.R) and the voltage V.sub.BE across the base/emitter of the transistor 21 to be greater than that of the forward voltages of the diodes 22, 23, a current I.sub.1 flows to hold the following relation, EQU I.sub.o =I.sub.R -I.sub.1
and I.sub.R is thus limited.
Even when transistors are coupled by a Darlington connection, diodes like those identified as 22, 23 are inserted between the base of a preceding stage transistor and the emitter of a final stage transistor. In this case, however, the number of diodes in series is selected to be greater by 1 or 2 than the number of the base/emitter junctions of the transistors.
As a practical matter, it is difficult to connect the overcurrent limiting diodes 22, 23 to external base and emitter terminals of a transistor when overcurrent protection is desired. Consequently, attempts have been made to incorporate the diodes into the same package containing the transistor. However, because the temperature coefficient of the resistor 24 is positive while that of the forward voltages of the diodes 22, 23 is negative, the temperatures of the diodes rise, causing their forward voltages to decrease, as the temperature of the transistor being energized rises, if the diodes are fitted onto the same heat radiation material as used for the transistor, or otherwise combined with the same semiconductor substrate for the transistor. On the other hand, the resistance value R of the resistor 24 increases because its temperature increases by the heat radiated from the transistor, thus excessively limiting the current. In order to prevent the temperature of the diodes from rising simultaneously as the temperature of the transistor rises, heat insulation should be provided between the diodes and the transistor on the base plate of the package, or otherwise the distance between them should be increased. However, the problem is that the package tends to become large, thus restricting the usage of such a device or increasing manufacturing cost.